This invention relates generally to ion pumps and is more particularly concerned with an ion pump construction in which gas leakage failure due to the phenomenon of ion etching through the vacuum housing component thereof is prevented and in which the latent period prior to the onset of ion pumping is accelerated.
Ion pumps are well known devices utilized for purposes of creating, maintaining and/or gauging vacuums in various technical systems. In general, an ion pump comprises: a vacuum housing composed of a non-ferromagnetic metal, usually a stainless steel or monel, and equipped with gas inlet means; a magnet system external of the housing for providing a magnetic field therewithin, said field defining a magnetic axis traversing the housing; an internal anode positioned substantially centrally within the housing aligned with the magnetic field and an internal cathode comprising opposed reactive sheet metal elements spaced from opposite ends of said anode and being interposed between the anode and the housing, said reactive elements overlying the magnetic axis. In use, usually after mechanically pumping the system to an at least somewhat reduced pressure, a high voltage is impressed across the anode and cathode, thereby to excite and ionize the gas admitted into the housing. Due to the influence of the magnetic field, the resulting positive ions are focussed as a core stream surrounding the magnetic axis of the field and flow to the negatively charged reactive sheet metal cathode elements to impinge upon, react with and thus become implanted as solid products of reaction therein. Generally, the cathode and housing are in electrical communication with one another and both are held at ground potential. Titanium is often the reactive metal employed for the construction of the reactive sheet metal cathode elements. Exemplary of such ion pump constructions is that disclosed in U.S. Pat. No. 4,460,317, to Robert H. Kern and Wilfred P. Levesque, entitled ION PUMP, issued Jul. 17, 1984, the entire disclosure of which is incorporated herein by reference.
One of the problems associated with ion pumps of the prior art is that, over time, the highly focussed ions contacting the reactive sheet metal cathode elements, particularly ionized noble gases concentrated as a core stream along the magnetic axis, ultimately etch holes through one or both said elements whereupon at least a portion of the streaming ions thereafter flow through the so holed reactive cathode elements to impinge upon and ultimately etch holes through the overlying walls of the vacuum housing. This last, of course, results in gas leakage through the vacuum housing and catastrophic failure of the pump. The problem is particularly acute where the gas to be pumped comprises one or more of the relatively readily ionized noble gas species, in other words, helium, neon, argon, krypton, radon or xenon. Noble gas ions tend to be highly reactive with respect to those metal compositions conventionally employed as materials of construction for the vacuum housing component. The predominant gas pumped in the many known ion pump applications is normal atmosphere, which, of course, contains such noble gas species. Accordingly, the foregoing mode of depletion and then subsequent failure of the housing represents a problem in the ion pump art.
Another operational problem in ion pumps of the prior art resides in the inherent occurrence of an initial, relatively lengthy, latent non-pumping period after impressing the high voltage across the anode and cathode components. This latent period is due to the fact that a certain amount of time is required to allow the high potential electric field across anode and cathode to experience initial ionization of a sufficient population of gaseous atoms and molecules to bring about the onset of a discharge in the feed gas and to thereby allow the pumping sputtering function to begin. This latent period is variable due to pressure and varying times to initial ionization. Where the application of the pump is in a system which is used repetitively throughout the work day, such as in a mass spectrometer of an analytical laboratory, these initial latent non-pumping periods can constitute troublesome delays, from seconds to periods approaching an hour.
In accordance with the present invention, the foregoing problems associated with ion pumps of the prior art have either been entirely eliminated or, at the least, substantially ameliorated.